The present invention relates to an electric antenna apparatus mounted on a vehicle such as an automobile or the like and, more particularly, to an electric antenna apparatus which uses the rotation of a motor to control the raising and lowering of an antenna rod and which detects an amount of rotation of this motor, thereby making it possible to control, for example, an amount of projection of the antenna rod.
An antenna mounted on an automobile for the reception of radio waves is ordinarily constituted so as to protrude and be mounted on the outside of the vehicle body. The extension and retraction of this antenna rod are properly controlled by a motor.
Such extension and retraction control of the antenna rod is executed by switching on the rotation of the motor for driving the antenna rod by using a switch provided near the driver's seat. For example, when a switch for the radio is turned on, a switch signal is detected, and a command is generated to drive the antenna rod upward or downward. When using the control to raise or lower the antenna rod in this way, a cam mechanism, which is controlled in correspondence with the rotation of the motor for driving the antenna rod, and a limit switch mechanism and the like are combined and set in accordance with the whole raising position and storing position of the antenna rod, and the specified position of the antenna rod is detected, thereby executing the stop control of the motor and the like. On the other hand, where the rotation of the motor is restricted for some reason, a large current flowing through the motor in the restricted condition is detected by a bimetal or the like. When an excessive motor current is detected by this bimetal, the power supply circuit of this motor is disconnected, thereby protecting the motor from being burnt and damaged.
However, in an apparatus with such an arrangement, a plurality of limit switches have to be arranged around the cam mechanism to correspond with the stop position, so a large space is required. In addition, the operating durability deteriorates due to the wear and tear of the contact mechanism, abrasion of the cam mechanism, and the like.
As an improved apparatus in consideration of the foregoing points, an antenna apparatus such as disclosed in the Official Gazette of Japanese Utility Model Application Laid-open No. 58-166106 has been considered. The means adopted in this apparatus is such that a rotary shaft of the reduction gear, which is driven by the output shaft of the motor, is provided with the contact lever, which is rotated together with this rotary shaft. A plurality of fixed contacts are arranged on the circumference which is coaxial with the rotary shaft of the contact lever, thereby allowing the contact of the contact lever to come into contact with one of the plurality of fixed contacts in response to an amount of rotation of the rotary shaft. Namely, the contacts of the contact lever sequentially come into contact with the plurality of fixed contacts in correspondence to the rotation of the motor. A pulse-like signal is generated whenever the rotated contact comes into contact with one of the fixed contacts. Therefore, by counting such a generated pulse signal, the amount of rotation of the motor, namely, the position of the antenna rod, can be detected.
However, nothing is set forth in the above Laid-open Official Gazette with respect to a practical means for processing and controlling the pulse-like signal generated in response to the rotation of the motor as described above; the only information disclosed in this Gazette states that the control circuit, constituting this processing means, is mounted in the vehicle interior.